The quantum-confined Stark effect (QCSE) has been extensively investigated for its use as an optical intensity modulator. The lowest-energy absorption edge of a semiconductor is a multiparticle system comprised of extended electron and hole states which are correlated through the Coulomb interaction. These states, known as Wannier excitons in semiconductors, have two properties which make them a noteworthy choice for intensity modulation. First, the absorption of these excitons can be quite large (around 3.times.10.sup.4 cm.sup.-1 under zero electric bias when the excitons are confined in a single dimension by a quantum well) and, second, the absorption peak can be shifted by applying an electric field across the active region. Thus, a wavelength region which had little or no absorption can be made to have quite high absorptive properties with an applied voltage. The ability to create intensity modulators follows directly. Quantum-well optical intensity modulators have been made in waveguides, in vertical reflection modulators, in vertical transmission modulators, and in vertical Fabry-Perot modulator configurations.
Among them, the asymmetric Fabry-Perot modulator (AFPM) has been proposed as one of the most promising devices for free-space optical interconnect. The free-space optical interconnects offer several advantages over traditional two-dimensional electronic circuits: complete connectivity, freedom from crosstalk, inherent impedance matching, high "pinout" density, and a lower ratio of drive power to data rate for relatively long connections. The AFPM architecture is well-suited to the optical interconnects for its surface-normal configuration, polarization independence, low insertion loss, low chirp, high contrast between on and off states, and VLSI-compatible drive voltage.
The types of modulators with the AFPM structure discussed in the previous works include reflection (reflective/absorptive) modulators and transmission (transmissive/absorptive) modulators. Five prior publications on the reflection modulators are shown below.
Pezeshki et al., "Optimization of modulation ratio and insertion loss in reflective electroabsorption modulators," (Appl. Phys. Lett., Vol. 57, No. 15, October 1990) shows theoretically that the maximum modulation ratio in Fabry-Perot reflective electroabsorption modulators for a given insertion loss is solely a function of the ratio of the maximum to minimum absorption.
Goossen et al., "Monolithic integration of normally-on and normally-off asymmetric Fabry-Perot modulators by selective anti-reflection coating," (SPIE Photonic Switching, Vol. 1807, 1992) integrates normally-on and normally-off high-contrast reflection modulators by selective AR coating.
Trezza et al., "High contrast asymmetric Fabry-Perot electroabsorption modulator with zero phase change," (Appl. Phys. Lett., Vol. 63, No. 4, July 1993) determines the wavelengths and biases at which there exist large absorption changes and zero refractive index changes relative to zero bias. At these wavelengths and biases, zero-chirp reflection modulators can be realized.
Trezza et al., "Zero chirp quantum well asymmetric Fabry-Perot reflection modulators operating beyond the matching condition," (J. Appl. Phys., Vol. 74, No. 12, December 1993) demonstrates asymmetric Fabry-Perot reflection modulators which operate beyond the matching condition and exhibit zero phase change (zero chirp) when switched.
The two prior publications on the transmission modulators are listed below.
Trezza et al., "Low-voltage, low-chirp, absorptively bistable transmission modulators using type-IIA and type-IIB In.sub.0.3 Ga.sub.0.7 As/Al.sub.0.33 Ga.sub.0.67 As/In.sub.0.15 Ga.sub.0.85 As asymmetric coupled quantum wells," (J. Appl. Phys., Vol. 74, No. 11, December 1993) theoretically analyzes and experimentally demonstrates the modulation of the optical transmission.
Lin et al., "Normally on GaAs/AlAs multiple-quantum well Fabry-Perot transmission modulator with ON/OFF contrast ratio &gt;7.4," (Appl. Phys. Lett., Vol. 66, No. 10, March 1995) reports a normal-incident normally on GaAs/AlAs multiple-quantum well Fabry-Perot transmission modulator.
A significant disadvantage of the prior art is that for both reflection modulators and transmission modulators, the incident photon energy is unrecoverable in one of the two switching states. In addition, the existing reflection modulators operate only with light impinging from one side of the wafer. Therefore, they can not be used as X-gates in which all logic functions can be decomposed. In the past, X-gates were realized using many CMOS gates or quantum electronics at millikelvin temperatures, both of which are incompatible for practical, efficient switching architectures. Optically, X-gates were made using meters of optical fibers which are clearly not compatible with dense packed device arrays.